Postfix help - mail to gmail, yahoo etc goes to spam

Some strange behavior is noted on my server and I require your help because I am lost and do not know how to start investigating this. My server is set up for debian 5 according to the tutorial.

When I send an email through kmail (local client) my email is delivered to spam folder for gmail or yahoo. On the other hand when I use the web interface at the server (squirrel mail) my email is delivered normally. This did not happen before and the only thing I did was restarting my server today.

edge thanks for your reply even though I was expecting a little bit more help from an answer. Please consider that everything was working just fine before so it is not a reverse DNS lookup issue nor anything else that I came across this forum.

The problem (??) is solved today and it is not related to any postfix configuration. I think it is a bug in my local client.